Temperature compensated reference cavity



June 11, 1957 v E'. R. MITTELMAN EIAL ,7

TEMPERATURE COMPENSATED REFERENCE CAVITY Filed Jan. 14, 1954 ENTORS Edward R. telmon 8| William D. Drummond.

United States Patent TEMPERATURE COMPENSATED REFERENCE CAVITY Edward R. Mittelman, Baltimore, Md., and William D. Drummond, Seattle, Wash, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 14, 1954, Serial No. 404,096

11 Claims. (Cl. 333-82) This invention relates to cavity resonators such as are used in wave guide systems, and has as an object to provide an improved means of compensating for the effect of temperature variations upon the resonant frequencies of such cavities.

The resonant frequency of a cavity resonator depends upon its physical dimensions which vary with temperature changes. This problem is well known, and a number of temperature compensating arrangements have been proposed. Such prior arrangements have had the dis advantages of being complicated and of adding to the size and cost of the resonators with which they were used.

This invention provides a relatively simple and inexpensive temperature compensating means for a cavity resonator, and which does not increase the size of the resonator. In one embodiment of the invention, a cylindrical cavity resonator has its walls formed of a metal having a low coefiicient of expansion. A coaxial inner conductor extends inwardly from one end of the resonator. A segmented ring of metal having a high coefiicient of expansion extends around the inner conductor within the region of high magnetic field between the inner conductor and the cylindrical wall around it. As the resonator is heated during operation, its walls expand and act to decrease the resonant frequency of the cavity. However, the segmented ring of higher expansion metal expands more towards the inner conductor than the walls of the resonator expand away from it. The decrease in volume in the region of high magnetic field raises the resonant frequency of the cavity, thus off-setting the decrease in frequency due to the expansion of the resonator walls.

This invention will now be described with reference to the drawings, throughout which like reference characters indicate like parts and in which:

Fig. 1 is a side elevation, in cross-section of a cavity resonator embodying this invention;

Fig. 2 is a sectional view along the lines 2-2 of Fig. 1;

Fig. 3 is a side elevation in cross section of a cavity resonator showing a modification of this invention, and

Fig. 4 is a sectional View along the lines 44 of Fig. 3.

The cavity resonator illustrated by the drawings is a fixed frequency, reference cavity of the type used in cavity comparator circuits for calibrating test cavities. The invention is not limited, however, to fixed frequency cavities since it applies equally as well to tunable cavities as will be apparent from the following description.

Referring in detail to Figs. 1 and 2, a cavity resonator is shown having a cylindrical wall it and two circular end walls 11 and 12 constructed from the alloy Invar or of another suitable metal having a low coeflicient of expansion. The end wall 11 has formed integral therewith, a cylindrical, co-axial reentrant conductor 13. In a tunable cavity this conductor could be a shaft slida- 'ble through the end wall 11 for varying the resonant frequency of the cavity.

2,795,764 Patented June 11, 1957 The segmented ring 14 of copper or another suitable metal having a high coetlicient of expansion, is placed around the inner conductor 13, with its outer surface in brazed and soldered to the inner surface of the cylindrical wall 10 of the cavity. The space between the inner conductor 13 and the segmented ring 14 is a region of high magnetic field.

Upon an increase in temperature, the slight expansion of the walls 10, 11 and 12 will cause a slight increase in the volume of the cavity, and a slight decrease in its resonant frequency. But, at the same time the segmented ring 14 will expand inwardly towards the inner conductor 13 more than the wall 10 expands away from it, and the inner conductor will expand slightly outwardly towards the segmented ring 14. This decrease in volume in the region having a high magnetic field, raises the resonant frequency of the cavity, thus compensating for the decrease in frequency caused by the expansion of the walls 10, 11 and 12.

Outward expansion of the segmented ring 14 is restrained by the much larger wall 10.

Upon a decrease in temperature, the walls 10, 11 and 12 will contract, decreasing the volume of the cavity and raising its resonant frequency. The segmented ring 14 will, however, contract more away from the inner conductor 13 than the wall 10 contracts towards it. At the same time, the inner conductor 13' will contract slightly away from the segmented ring, 14. The resulting increase in volume in the region having a high magnetic field will lower the resonant frequency of the cavity and will compensate for the decrease in frequency caused by the contraction of the walls 10, 11 and 12.

By suitable relations between the dimensions of the walls of the cavity, of the segmented ring 14 and the inner conductor 13, any change in the dimensions of the resonator caused by temperature changes, can be exactly compensated for. If desired, over or under compensation could be provided.

As is conventional, suitable entrance apertures which are not illustrated, for coupling the resonator to a wave guide system, could be provided in its walls.

Referring in detail to Figs. 3 and 4, there is shown a resonant cavity similar to that shown in Figs. 1 and 2 except for the modification in the segmented ring 14 and mounting thereof. In Figs. 3 and 4, a plurality of apertures 22 are provided in the cylindrical wall 10. In the specific embodiment, the centers of the apertures 22 lie in a plane perpendicular to the cylindrical wall 10. A plurality of pins 23 are provided of suitable dimensions so that a pin 23 may be inserted into each of the apertures 22 so as to project into the cavity and terminate substantially the same distance from the inner conductor 13. The pins 23 are made of a material of a high coefficient of expansion compared to the material of the body 10. The pins 23 may be secured to the body 10 within the apertures 22 by suitable means, such as brazmg.

The structure of the device shown in Figs. 3 and 4 is more suitable for manufacturing. The operation of the device shown in Figs. 3 and 4 is similar to that of Figs. 1 and 2 with the plurality of pins 23 serving the same purposes as the segmented ring 14.

While only two embodiments of the invention have been described for the purpose of illustration, it should be understood that the invention is not limited to the exact apparatus and arrangement of apparatus illustrated, since modifications thereof may be suggested by those skilled in the art, without departure from the essence of 0 the invention.

We claim as our invention: 1. A cavity resonator comprising a cylindrical tube closed at one end, said tube being of material having a relatively low coefiicient of expansion, a coaxial conductor extending from said end of said tube inwardly of the tube and terminating substantially short of the other end of the tube, and a segmented ring of material having a relatively high coeflicient of expansion extending within said tube around said conductor.

2. A cavity resonator as claimed in claim 1 in which the segmented ring is supported in contact with the inner surface of the tube.

3. A cavity resonator as claimed in claim 2 in which the inner conductor is formed integrally with said end of said tube.

4. Acavity resonator as claimed in claim 1 in which the inner conductor is formed integrally with said end of said tube.

5. A cavity resonator comprising a cylindrical outer wall around the cavity, an end wall at one end of the cavity, a conductor extending from said end wall inwardly of the cavity and terminating substantially short of the other end of the cavity, said outer wall being of material having a relatively low coefficient of expansion, and a plurality of members of material having a relatively high coefiicient of expansion around said conductor, between said conductor and said outer wall.

6. A cavity resonator as claimed in claim 5 in which the outer-surface of the members are in contact with the outer wall.

7, A cavity resonator as claimed in claim 6 in which there is an end wall at the other end of the cavity, and in which the end walls are of material having a relatively low coefiicient of expansion.

8. A cavity resonator as claimed in claim 5 in which there is an end wall at the other end of the cavity, and in which the end walls are of material having a relatively low coefiicient of expansion.

9. A cavity resonator as claimed in claim 1 in which the material closing the wheat the said one end has a relatively low coefiicient of expansion.

10. A cavity resonator as claimed in claim 1 in which the tube is closed at the other end, and in which the material closing the ends of the tube has a relatively low coefficient of expansion.

11. A cavity resonator comprising a cylindrical outer Wall around the cavity, an end Wall at one end of said References Cited in the file of this patent UNITED STATES PATENTS 2,142,630 Conklin Jan. 3, 1939 2,181,871 Conklin Dec. 5, 1939 2,374,810 Fremlin May 1, 1945 2,507,426 Turney May 9, 1950 2,608,671 Fremlin et a1 Aug. 26, 1952 

